Drove to and back from Calgary to Vegas on two separate occasions last year.  24hour straight drive.  First trip by myself and second trip with a company.  Was not well rested, took in minimal fluids and ate minimally.  I was pretty much alert the whole time, as I do enjoy long commutes, beautiful scenery, winding roads, and singing to my favourite tunes helped me get there 90% of the time.  The other 10% I relied on caffeine and a 30 minute nap.  
Doing the daily grind driving between work (had to work 2 full time jobs back to back back then) was the most difficult I find.  When you're working you don't notice it, but once you stop and make the drive to your next job that's when it hits you hard.  On two occasions I had to wind down the window and have wind blow into my face on the highway and couldn't wait to get off the highway.  Yeah, it was bad.